The present invention pertains to semiconductor based image sensors and more particularly to Active Pixel image sensors (APS) having high sensitivity and increased dynamic range.
APS are solid state imagers wherein each pixel contains both a photosensing means and at least one other active component, creating a charge that is converted to a signal (either a voltage or current signal). The signal represents the amount of light incident upon a pixel photosite. The dynamic range (DR) of an imaging sensing device is defined as the ratio of the effective maximum detectable signal level, typically referred to as the saturation signal, (Vsat), with respect to the rms. noise level of the sensor, ("sgr"noise) This is shown in Equation 1.
Dynamic Range=Vsat/"sgr"noisexe2x80x83xe2x80x83Equation 1:
Image sensor devices such as charge coupled devices (CCD) that integrate charge created by incident photons have dynamic range limited by the amount of charge that can be collected and held in a given photosite, (Vsat). For example, for any given CCD, the amount of charge that can be collected and detected in a pixel is proportional to the pixel area. Thus for a commercial device used in a megapixel digital still camera (DSC), the number of electrons representing Vsat is on the order of 13,000 to 20,000 electrons. If the incident light is very bright and creates more electrons that can be held in the pixel or photodetector, these excess electrons are extracted by the anti-blooming means in the pixel and do not contribute to an increased saturation signal. Hence, the maximum detectable signal level is limited to the amount of charge that can be held in the photodetector or pixel. The DR is also limited by the sensor noise level, "sgr"noise. Due to the limitations on Vsat, much work has been done in CCD""s to decrease "sgr"noise to very low levels. Typically, commercial megapixel DSC devices have a DR of 1000:1 or less.
The same limitations on DR exist is for APS devices. The Vsat is limited by the amount of charge that can be held and isolated in the photodetector. Excess charge is lost. This can become even more problematic with APS compared to CCD due to the active components within the pixel in the APS, limiting the area available for the photodetector, and due to the low voltage supply and clocks used in APS devices. In addition, since APS devices have been used to provide image sensor systems on a chip, the digital and analog circuits used on APS devices such as timing and control and analog to digital conversion, that are not present on CCD""s, provide a much higher noise floor on APS devices compared to CCD. This is due to higher temporal noise as well as possibly quantization noise from the on-chip analog to digital converter.
In commonly assigned U.S. patent application Ser. No. 09/426,870, Guidash explains the prior art approaches to extending dynamic range of APS devices, and discloses a new invention to extend dynamic range by collection of the charge that blooms from the photodetector. While that approach does provide extended dynamic range with a small pixel, it has the potential disadvantage of spatial variation of the photodetector saturation level contributing to fixed pattern noise in the sensor, and does not increase the sensitivity of the sensor.
Prior art APS devices also suffer from poor sensitivity to light due to the limited fill factor induced by integration of active components in the pixel, and by loss of transmission of incident light through the color filter layer placed above the pixel.
From the foregoing discussion it should be apparent that there remains a need within the prior art for a device that retains provides extended dynamic range while retaining low fixed pattern noise, small pixel, and high sensitivity.
According to the present invention, there is provided a solution to problems of the prior art. In the present invention, the floating diffusion region in each pixel is used as a separate photodetector region to provide extended dynamic range and high sensitivity.
A first embodiment of the present invention provides extended dynamic range and higher sensitivity by utilizing a floating diffusion region without a light shield provided in each pixel as a separate photodetector region. During integration of signal charge on the photodetector, charge is also collected on the floating diffusion in proportion to the light incident on the floating diffusion region. In prior art devices the floating diffusion region is used as the charge to voltage conversion node, as an overflow drain for the photodetector during integration, or as a charge storage region for global frame capture. As a result, the floating diffusion region is either shielded from incident light, or is held in a reset mode to prevent the accumulation of charge resulting from light incident on or near the floating diffusion region, and to drain the blooming charge from the photodetector region. In the present invention charge is integrated on the floating diffusion in proportion to the amount of light incident upon the floating diffusion for a period of time that is controlled independently from the photodetector integration time. The charge integrated on the floating diffusion is then read out separately from the charge integrated on the photodetector. In this first embodiment the photodetector and floating diffusion in a given pixel are covered by the same color filter, or are both not covered by any color filter.
A second embodiment of the present invention provides extended dynamic range and high sensitivity to incident light by utilizing the first embodiment with a different or separate color filter for the photodetector and floating diffusion region in a given pixel. This provides a signal charge associated with 2 colors per pixel.
According to the present invention, an active pixel sensor device that significantly increases the dynamic range and sensitivity of the device, and can be used in current system designs is provided by: an X-Y addressable imager having a plurality of the pixels within the X-Y addressable imager with a photodetector within each of the plurality of pixels configured to sense a first bandwidth of light; a sense node within each of the pixels configured to sense a second bandwidth of light; a reset mechanism operatively configured to the photodetector and the sense node to allow resetting each of the photodetector and the sense node to a predetermined potential, the sense node being formed such that it does not have a light shield allowing the sense node to act as a second photodetector; and a transfer mechanism within each of plurality of pixels configured to transfer charge from the photodetector to the sense node. The first and second bandwidths can be different or the same depending upon design choices. The X-Y addressable imager is envisioned as comprising a system with a first storage mechanism to store a signal associated with charge accumulated on the sense node, a second storage mechanism to store a signal associated with charge accumulated on the photodetector and a timing circuit for controlling the integration and transfer timing of the sense node and the photodetector for each of the plurality of pixels.
The invention has the following advantages. It provides for extending the dynamic range and sensitivity of a sensor that can easily be employed within current sensor and pixel designs with little or no modification. Small pixels with high fill factor can provide separate signals from 2 colors per pixel.